Mobile networks such as 2G (GSM) or 3G (UMTS) telecommunications networks have an active state of communication with their mobile terminals and an inactive/idle state of communication with their terminals. When in the active state, as the mobile terminals move between different cells of the network, the communication session is maintained by performing a “handover” operation between the cells. In the inactive/idle state, as a mobile terminal moves between different cells of the network the mobile terminal performs “cell reselection” to select the most appropriate cell on which to “camp” in order that the mobile terminal can be paged by the network when mobile terminating data is destined for that mobile terminal.
Calculations to determine whether to perform a handover from one base station to another base station are performed by the network, whereas calculations whether to perform cell reselection are performed by the mobile terminal.
The frequency with which a mobile terminal checks to determine whether any paging messages are transmitted by the network is the “DRX Cycle Length”. The DRX Cycle Length value is transmitted by the network to the mobile terminal and is the same for all mobile terminals in a particular group of cells (in a particular registration area, routeing area or location area). The DRX Cycle Length is intended to reduce the terminal battery consumption, and thus operators would normally set this as long as possible whilst considering paging message delay.
In the idle/inactive state, cell reselection is performed in accordance with measurements of the signal strength (or the like) of the cell on which the mobile terminal is currently camped and of neighbouring cells. These measurements are required to be performed at a minimum frequency determined by a cell measurement cycle length (CMCL). It is a limitation of known mobile telecommunications systems that the cell measurement cycle length is defined relative to the DRX cycle length (it is a divisor of the DRX cycle length). Thus, when the DRX cycle length is transmitted to a mobile terminal by the network, this also sets the cell measurement cycle length. However, the setting of the cell measurement cycle length in this way can be disadvantageous. The cell measurement cycle length should be shorter if a mobile terminal has a fast moving mobility state and can be relatively long when the mobile terminal is in a slow moving mobility state. However, in the prior art, the cell measurement cycle length is set only in dependence upon the DRX cycle length.
In the idle/inactive state the mobility state of a mobile terminal can be used to optimise cell reselection parameters. Similarly, in the active state, the calculated mobility state can be used to optimise the handover parameters. In the idle/inactive state the mobility state is determined by the mobile terminal, whereas in the active communication state, the mobility state is calculated by the network. The cell reselection parameters and handover parameters can be set optimally by using a history of mobility state measurements. However, it is difficult to build a history of mobility state measurements in either the active state or inactive state in the prior art if the mobile terminal frequently moves between the active and inactive states.